The police, who were also guarding key sites, including railway and subway stations, department stores and journalism offices, were said to be narrowing their search for the brothers to northern France, where the armed men broke into a gasoline station to get food and later abandoned one of the cars they had used in their getaway from Paris on Wednesday.
The sighting of the men at the station and the discovery of the car, in the town of Villers-Cotterets, in Picardy, captivated a nation that seemed to come together, at least for a moment of silence at noon on a rare official day of national mourning, to defend French values like freedom of the press and religious tolerance.
Thousands wore stickers reading "Je suis Charlie," referring to the magazine whose editors and most prominent cartoonists were killed, Charlie Hebdo. National television ran constant live coverage of the manhunt for the fugitives, Said and Cherif Kouachi, 34 and 32.
The gunmen killed their victims and wounded 11 more people with precision and calm, and were heard on videos shouting "Allahu akbar!" and "We have avenged the Prophet Muhammad! We have killed Charlie Hebdo!" The men said that they were acting on behalf of al-Qaida in Yemen, according to a witness. Two US officials said Thursday that the brothers had ties to al-Qaida's affiliate in Yemen, but the officials declined to say whether that meant the suspects had been in communication with the group or had actually traveled there or received training.
Questions were raised about why the police and security services, who had known about the brothers - one of whom had spent time in jail for jihadist activities - had failed to disrupt the attack.
The moment of silence here was widely respected. But some worried that the sense of national solidarity may not last, given the shock these killings have delivered, which Arash Derambarsh, a publisher and politician, compared to Sept. 11 in the United States. There, Muslim radicals of al-Qaida struck the symbols of US economic and political power; here, the target - the small, often vulgar satirical magazine, Charlie Hebdo, was an important cultural symbol of French secularism, liberty and license.
"This is a war between freedom of speech and our civilization and those who want to kill it," Derambarsh said.
Charlie Hebdo announced that despite the loss of so many of its most talented people - cartoonists who have been famous in France for a generation - it would publish as scheduled next Wednesday, and rather than print the usual 60,000 copies, would print 1 million.
But the mood of defiance was fragile. Thursday morning, there was panic after another gunfight, when a woman police officer was killed and a city employee was wounded near a subway station just south of the capital.
The incident seemed unrelated, the police said, and announced two arrests. But there were other isolated events. There was an explosion at a kebab shop in eastern France, with no casualties reported, and two mosques were fired at, prosecutors said.
In a sign of how the attack was already spilling over into French political debates, Marine Le Pen, leader of the far right National Front, on Thursday called for a national referendum on whether to reinstitute the death penalty.
"The Islamists have declared war against France," she told France 2. Le Pen said that she had not been invited to a "unity rally" scheduled for Sunday to which other political leaders have been invited.
"Things are clear from now on, the masks fall off," she told Le Monde newspaper. "National unity is a pathetic political maneuver."
A loud debate on immigration, already a central and emotive issue in France and across Europe at a time of economic malaise, was expected to become even more central following attacks that had underlined the sometimes lethal clash between Western values and religious extremism.
The French government moved quickly to try to capture the fugitives and reassure the nation, announcing numerous lower-level arrests and saying that at least five planned terrorist attacks had been thwarted in the last 18 months. The alert level was raised to its highest in northern France, as well as in the capital. The police had a break when one of the brothers, Said, left his identity card in the first car used by the gunmen, which was abandoned Wednesday evening after a crash and reportedly contained Molotov cocktails and jihadist banners.
The authorities seemed to catch another break when the men robbed the gasoline station, wearing masks and waving Kalashnikovs, according to the station's manager. Xavier Castaing, a spokesman for the Paris Police, said two men fitting the description of the suspects had been spotted in Villers-Cotterets, reportedly driving a gray Renault Clio, the same vehicle model that the two suspects had hijacked Wednesday.
There was considerable interest in Cherif Kouachi, who had been arrested in 2008 for being part of a terrorist cell recruiting French jihadists to fight in Iraq. He served 18 months in prison and was thought to have been further radicalized there. Both brothers were on the radar of the police and security services, but they had not picked up this well-planned attack, just as they failed in 2012 to stop Mohammed Merah from killing seven people in southwestern France, on behalf of al-Qaida, he said, even though they had followed him after his own stints in prison.
There are not enough police and security officials to keep full monitoring on everyone who goes through prison, said Jean-Charles Brisard, head of the French Center for Analysis of Terrorism, who had spoken to French security officials. The authorities had Chérif Kouachi under surveillance "for a period of time, but then they judged that there was no threat, or the threat was lower, and they had other priorities," he said.
Given the 1,000 to 2,000 French citizens who have traveled to Iraq and Syria to fight, perhaps 200 of them having returned, "it's a problem of resources," Brisard said. "To follow a person 24 hours a day you need at least 20 people. And you cannot impose surveillance on everyone; even legally it's impossible."
A third suspect, Hamyd Mourad, 18, turned himself in at a police station in Charleville-Mezieres, about 145 miles northeast of Paris.
At noon, on this national day of mourning for the worst terrorist attack in France since the Algerian war, bells rang; schools stopped classes; corporate boardrooms truncated meetings. At mosques, people bowed their heads. Some electronic road signs displayed the words, "Je suis Charlie."
At Notre Dame Cathedral, pedestrians wept as dozens stood silent on a gray and rainy day to pay tribute to the victims. Dozens lay flowers in front of Charlie Hebdo's headquarters. Through the vigils and tributes, some held pencils, a symbol of support for press freedom. There was a palpable sense of determination that France and its vaunted Republican values of free speech and freedom of the expression would not be subverted by religious extremism.
The night before, perhaps 100,000 people or more gathered all over France to show defiance and mourn the blow to their sense of their country.
And many Muslim clerics and spokesmen added their voices to the sense of outrage, decrying the killings as antithetical to legitimate Islam.
Abdennour Bidar, a French Muslim and professor of philosophy, said on Arte television that the killers "do not deserve the name of Muslims." In the name of Islam, he said, he would not allow Islam to be "instrumentalized, stolen by these people who say that they are avenging the Prophet. It's a disgrace, an infamy, a lie."
The assault against Charlie Hebdo has threatened to fan resentment and suspicion of France's Muslim population, the largest in Europe. Authorities said that at least two Muslim locations had been targeted overnight, and that explosives had been lobbed at a mosque in Le Mans, a city west of Paris early Thursday. News reports said that a Muslim prayer hall in the Port-la-Nouvelle area in southern France had also been targeted and shot at. They said no one had been injured.
Prime Minister Manuel Valls called for citizens not to submit to hatred. Charlie Hebdo must continue its "right to make fun of everyone," he said. "There are no borders constraining the freedom of expression and thought."
At a Mass in Rome, Pope Francis prayed for the victims. He also offered prayers " for the perpetrators of such cruelty, that the Lord might change their heart."
One cartoonist called Matt at London's Daily Telegraph paid tribute to those killed by drawing a cartoon of two masked gunmen outside Charlie Hebdo's offices conferring with each other. "Be careful, they might have pens," reads the caption.